The existence of a static electric field in the earth's atmosphere has long been recognized. More recently, this static electric field was determined to exhibit a vertical potential gradient, the change in potential per meter decreasing with increasing altitude. At any altitude, however, equipotential lines and planes are essentially horizontal, especially in a localized area. The ability to define a horizontal equipotential line or plane in the atmosphere would provide a reference for aircraft stabilization, gyroscopic drift correction, or clean air turbulence detection inter alia. Additionally, detection of tilted equipotential planes or lines in the vicinity of various physical objects provides a reference for obstacle avoidance apparatus.
In particular, the present invention provides related methods and preferred means for utilizing the earth's static field existing therein, to obtain a reference signal which can be employed to control both pitch and roll axes stabilization of airborne vehicles. In a general sense, the invention provides for continuously defining an equipotential line or plane within the earth's vertical static potential gradient. Definition of this equipotential line or plane is indicated by a simple DC voltage signal produced by the preferred apparatus of the invention, this signal being usable to accomplish a variety functions. The invention further relates to a general method for continuously defining an equipotential line or plane in the earth's vertical static potential gradient. The method comprises immersion of at least two voltage sensing probes in the earth's static potential gradient and measuring the difference in the static voltage sensed by the two probes. If the two probes sense the same voltage, i.e., the difference between the sensed voltage is zero, then the probes usually lie on a horizontal line of equal potential due to the vertical nature of the earth's static potential gradient. Similarly, a horizontal equipotential plane may be defined by two pairs of voltage sensing probes or by three probes, one of which is referenced to the electrical center of the other two probes. Nonhorizontal equipotential planes and lines may be sensed in similar fashion in those areas where the vertical gradient is distorted due to physical objects or ionization sources.
The preferred apparatus of the present invention comprises voltage sensing probes (which probes may be comprised of radioactive material in order to increase the electrical contact thereof with the atmosphere) and a differential static amplifier. The amplifier receives the sensed potentials from the probes and measures the difference in the sensed potentials to produce a voltage difference signal which may be utilized to control a servomechanical or other apparatus. The voltage sensing probe/differential static amplifier combination is referred to hereinafter as a differential static voltmeter. The actual or effective combination of two differential static voltmeters can essentially define an equipotential plane in the earth's vertical static potential gradient. Since this equipotential plane is defined within a relatively small space, the plane is essentially horizontal and may be used as a reference plane for aircraft stabilization inter alia.
If the sensing probes described above are attached to the wingtips of an aircraft or essentially along the transverse axis thereof, then the voltage difference between the probes measured by the differential static amplifier is a function of the roll angle of the aircraft. Similarly, the voltage difference sensed by a pair of sensing probes disposed essentially along the longitudinal axis of an aircraft is a function of the pitch angle of the aircraft. The differential voltage output signals thus produced for the pitch and roll orientations of the aircraft can be used to drive a servomechanical system to maintain the aircraft in level flight relative to the surface of the earth. The sensing probes need not be exactly aligned with or parallel to the transverse or longitudinal axes of the aircraft in order to produce useful information. Similarly, the probes need not be positioned at exactly the same elevation relative either to the earth's surface or to the horizontal axis of the aircraft.
The invention also relates to a method and apparatus for measuring the roll rate or pitch rate of an airborne vehicle. The apparatus used to sense rate comprises a pair of conductive wires disposed essentially along or parallel to the transverse axis or longitudinal axis of the aircraft. Rotation of these wires in the earth's static electric field generates a current in the wires. The generated current is measured by a current meter, the intensity of the current being proportional to the roll rate or pitch rate of the vehicle.
Thus it is an object of the invention to provide a method and apparatus for continuously defining an equipotential line or plane in the earth's static electric field.
It is another object of the invention to provide differential voltage sensing apparatus aboard an airborne vehicle, the differential voltage sensed by said apparatus being a function of the roll or pitch attitude angle of the vehicle.
It is a further object of the invention to provide a differential static current meter for determining the roll rate or pitch rate of an airborne vehicle by measuring the current induced in a pair of conductive wires rotated in the earth's static potential gradient.
Further objects and advantages of the invention ill become more readily apparent in light of the following detailed description.